1. Field of the Invention
This invention relates to laser printers and, more particularly, to a constant power driver for a laser diode of a laser printer, which driver operates to eliminate laser shadow on a printed page.
2. Description of the Prior Art
Laser printers are utilized to provide high quality hard copy of computer generated documents at high speed and moderate to high resolutions. The most advanced of laser printers can provide both high resolution and high speed.
Basically, a laser printer is a relative of the common office electrophotographic copier. The light output of a laser diode is modulated by graphics or text data from a computer. An optical lens system is used to focus the light beam output to a spot on a photosensitive surface similar to the photoconductor drum of the electrophotographic copier. The spot is caused to sweep across the photosensitive surface in a raster fashion by a rotating and/or oscillating mirror. Computer or video raster information modulates the light beam as the photosensitive surface moves in a direction perpendicular to and under the sweeping light beam. The sweeping and modulated light beam causes the formation of an electrostatic charge image on the drum. This image can then be developed using a toner and transferred to paper. The toner is then fused to the paper, usually with heat and pressure as in a conventional electrophotographic copier. An example of such a laser printer is disclosed in U.S. Pat. No. 3,867,571 of Starkweather, et al.
The laser diode in a laser printer is, as its name implies, a semiconductor device and generally includes P and N regions which may have a dielectric waveguide therebetween. The dielectric waveguide generally has two parallel, polished ends, between which radiation is reflected. The rear surface includes a layer of a highly reflective metal, whereas the front surface transmits light and may be merely the cleaved end of a crystal.
The fundamental light-producing mechanism in the semiconductor is the recombination of excess conduction band electrons and valence band holes. At low current densities the electrons recombine with the holes, spontaneously emitting radiation in all directions. At higher currents an inverted population is created. Lasing action occurs when a light pulse makes a round trip between the front and rear surfaces within the waveguide without attenuation, and escapes from the front surface of the waveguide.
As in an LED (light emitting diode), in order to emit light, the pn junction is forward biased. Also, as in most semiconductor devices, operation is dependent upon junction temperature. As the junction temperature in a laser diode rises, light output decreases.
In a typical laser printer the laser emits light onto the electrophotographic drum for the white portions of the printed page, and is turned off for the dark portions of the printed page. The junction cools rapidly when turned off, such that after writing the "dark" portions of the pate, there will be a shadow effect on the next written white portion since it takes a finite amount of time for the junction to heat up. Conversely, this is not a problem in the other case because the laser diode is completely cut off in the write dark portions and therefore not sensitive to laser junction temperature.
Prior art attempts to overcome these disadvantages have only been partially successful. In one method known in the art, the laser is simply left on at all times; but, the amount of power to the laser is increased when it is desired to write a "white" area. This provides only a partial solution since the temperature of the junction still varies as the power to the laser is varied to write the white and black portions of a page. Further, this technique reduces the contrast of the printed page since the usable dynamic range of the laser diode is decreased. This is a particular disadvantage when writing graphics with a gray scale.
Another technique known in the art for reducing laser shadow is the use of feedback. A sensor is used to detect either laser light output or junction temperature and the power to the diode is adjusted accordingly. In a high speed, high resolution laser printer, however, each picture element ("pel") may have a write time of only about 50 nanoseconds (ns). Thus, feedback is not widely used because the circuitry generally available, at non-prohibitive prices, is not fast enough.